21's or 22's...?
02-14-2014, 10:00 PM
#17
Instructor
02-14-2014, 10:47 PM
#18
Rennlist Member
The larger the diameter wheel and tire combination, the heavier it is, increasing rotational inertia (degrades acceleration & braking), and increasing unsprung weight; the only advantage is in the tire's transient response where the shorter/stiffer sidewall brings improvement. Enlarging wheel diameter to allow larger brakes is one other factor to be considered in the equation. The trade-off point will depend on how all these factors interact in each individual model's applications.
All the rest is for show.
02-14-2014, 11:37 PM
#19
Instructor
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Plenty of people run 21" wheels and they work just fine.
Many aspects about modern Porsches arent purely for performance or driving dynamics. Leather, power seats, power windows, electric steering, PDCC, etc... 21" wheels dont degrade performance that much, and unless one is using their car to near its full potential it doesnt really matter. Most owners dont have enough skill or desire, so building a street car makes more sense.
It's annoying enough that 20" wheels are standard on the Carrera S, and offered on the even smaller Boxster/Cayman S. I can't imagine that a 20" wheel was a pure engineering/driving dynamics decision on the part of Porsche. They're simply giving in to popular trends for street looks. If 20" wheels were go-fast parts, they'd be on all the Rolex series (sorry, Tudor) and Continental Challenge cars.
21" and 22" wheels would essentially degrade much of the performance that one pays for in getting a 911--slower acceleration, longer braking distances, worse handling...
I find it ironic that my car will have the 20" OEM wheels as its winter setup with winter tires, while the summer/track setup will be 19" wheels...or perhaps not ironic at all.
21" and 22" wheels would essentially degrade much of the performance that one pays for in getting a 911--slower acceleration, longer braking distances, worse handling...
I find it ironic that my car will have the 20" OEM wheels as its winter setup with winter tires, while the summer/track setup will be 19" wheels...or perhaps not ironic at all.
02-15-2014, 01:52 AM
#20
Shorter sidewalls actually have worse lateral stability. Search around you'll find diagrams of the tire contact patch with low and high aspect ratio tires. As the aspect ratio gets lower the contact patch becomes narrower front to back and wider side to side. So a lower aspect ratio (shorter sidewall) tire has less directional stability, but this also means its easier to change direction, and the tire feels more responsive.
Now if we continue doing this down to a zero sidewall tire the contact patch is going to be a line. But a line has no area, and we can't have a 'patch' without area. So there's a limit. And the patch is both side to side and front to back. So we got a bit of a problem. Because, what we want is a low sidewall for responsiveness, but we also need to keep the contact patch from getting too short front to back. Gosh, if only there were a way to make the sidewalls a little lower without losing too much contact patch.
Right about here I imagine an engineer raising his hand, "increasing wheel diameter will accomplish this. I don't know if the market will accept it, but it will perform the desired function."
Won't that increase rotational inertia? "A little. But look how much power we have now. And have you seen those brakes? Oh, and you do know about the ride, don't you?"
The ride?
"Yes. Increased diameter improves the ride. The wheel goes over bumps better. Compensates for the lower sidewall."
Now don't quote me on this, its not on tape. But I think it does at least show that it is possible to imagine there might be something other than marketing behind Porsche re-engineering the 911 for larger wheels.
02-15-2014, 02:23 AM
#21
Race Director
I think Viken needs to weigh in on this issue.
02-15-2014, 03:03 AM
#24
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naturally, a tire able to bleed off the most heat while maintaining maximum cornering will occur by increasing it's surface area.... but could most of us be coaxed into a peaceful coexistence with the roadway stammering along on 19" tires?
of that, you can be sure.
can someone translate Walter's explanation at 2:40;
www.youtube.com/watch?v=RClMSgXdrvM
searching for an amazing video of Rohrl @ 140+ mph on an extremely narrow, rural, twisty...
on just his old 19" Pilot Sport tires.
this isn't it (but getting warmer)....
www.youtube.com/watch?v=OaRtdZ428HI
still can't find it, but he was getting along something like this.... (on 19" PS2s).
of that, you can be sure.
can someone translate Walter's explanation at 2:40;
www.youtube.com/watch?v=RClMSgXdrvM
searching for an amazing video of Rohrl @ 140+ mph on an extremely narrow, rural, twisty...
on just his old 19" Pilot Sport tires.
this isn't it (but getting warmer)....
www.youtube.com/watch?v=OaRtdZ428HI
still can't find it, but he was getting along something like this.... (on 19" PS2s).
Last edited by odurandina; 02-15-2014 at 04:18 AM.
02-15-2014, 03:05 AM
#25
I have 26" dubs on order 
02-15-2014, 11:43 AM
#27
Burning Brakes
You're right on about form following function. In their blood by definition, because its in their DNA. Evidently not everyone knows enough about Porsche to understand this.
Shorter sidewalls actually have worse lateral stability. Search around you'll find diagrams of the tire contact patch with low and high aspect ratio tires. As the aspect ratio gets lower the contact patch becomes narrower front to back and wider side to side. So a lower aspect ratio (shorter sidewall) tire has less directional stability, but this also means its easier to change direction, and the tire feels more responsive.
Now if we continue doing this down to a zero sidewall tire the contact patch is going to be a line. But a line has no area, and we can't have a 'patch' without area. So there's a limit. And the patch is both side to side and front to back. So we got a bit of a problem. Because, what we want is a low sidewall for responsiveness, but we also need to keep the contact patch from getting too short front to back. Gosh, if only there were a way to make the sidewalls a little lower without losing too much contact patch.
Right about here I imagine an engineer raising his hand, "increasing wheel diameter will accomplish this. I don't know if the market will accept it, but it will perform the desired function."
Won't that increase rotational inertia? "A little. But look how much power we have now. And have you seen those brakes? Oh, and you do know about the ride, don't you?"
The ride?
"Yes. Increased diameter improves the ride. The wheel goes over bumps better. Compensates for the lower sidewall."
Now don't quote me on this, its not on tape. But I think it does at least show that it is possible to imagine there might be something other than marketing behind Porsche re-engineering the 911 for larger wheels.
Shorter sidewalls actually have worse lateral stability. Search around you'll find diagrams of the tire contact patch with low and high aspect ratio tires. As the aspect ratio gets lower the contact patch becomes narrower front to back and wider side to side. So a lower aspect ratio (shorter sidewall) tire has less directional stability, but this also means its easier to change direction, and the tire feels more responsive.
Now if we continue doing this down to a zero sidewall tire the contact patch is going to be a line. But a line has no area, and we can't have a 'patch' without area. So there's a limit. And the patch is both side to side and front to back. So we got a bit of a problem. Because, what we want is a low sidewall for responsiveness, but we also need to keep the contact patch from getting too short front to back. Gosh, if only there were a way to make the sidewalls a little lower without losing too much contact patch.
Right about here I imagine an engineer raising his hand, "increasing wheel diameter will accomplish this. I don't know if the market will accept it, but it will perform the desired function."
Won't that increase rotational inertia? "A little. But look how much power we have now. And have you seen those brakes? Oh, and you do know about the ride, don't you?"
The ride?
"Yes. Increased diameter improves the ride. The wheel goes over bumps better. Compensates for the lower sidewall."
Now don't quote me on this, its not on tape. But I think it does at least show that it is possible to imagine there might be something other than marketing behind Porsche re-engineering the 911 for larger wheels.
No doubt, Porsche pays attention to styling...but after performance targets are met, and then not to their detriment.
02-15-2014, 04:08 PM
#28
Oh yeah, its way more complicated and sophisticated than most of us imagine. Great example with the bikes. (And tip of the hat to you- I tried velo, took track classes, its hard!) Remember that video where Preuninger (and this time I'm sure its Preuninger!) is talking about PDK weighs more, but we took x kg out of the engine? Same thing. And you were right in the beginning, its in their blood. Porsche spent millions engineering a whole new car, with new suspension, wheelbase, track and body, only to slap on some rims cuz that's phat, homes? I don't think so! 
